Problem: The scale on a map is 4cm : 9km. If two cities are 36cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 4cm on the map corresponds to an actual distance of 9km. A distance of 36cm on the map is the same as 9 $\cdot$ 4cm. The actual distance between the two cities is 9 $\cdot$ 9km, or 81km.